Synthesis, structure and insertion reactivity of Lewis acidic 9-aluminafluorenes.

9-Aluminafluorenes have only been sparingly investigated and their properties still remain largely unexplored. Herein, we report the synthesis of five aluminafluorene derivatives with a diverse array of aluminium substituents and probe their Lewis acid properties and reactivity. We show that 9-bromo-9-aluminafluorene readily forms Lewis acid–base adducts with N-heterocyclic carbenes (NHCs), cyclic (alkyl)(amino)carbenes (CAACs) and pyridines and that it undergoes a selective ring expansion reaction with the iminoborane tBuNBMes to give a seven-membered ring, which can be viewed as a boron–nitrogen analogue of alumepins.